Traditionally, vehicles have a 4×2 system where only two wheels at one axle (either the front or the rear) can provide driving torques. Recently, many SUVs have a 4×4 system which can distribute power to all four wheels instead of two wheels. The benefit of the 4×4 system is easy to understand. Such a system has the potential to double the amount of longitudinal force over a conventional 4×2 system if the traction potentials of the tires exceed the driving torques applied. There are three types of 4×4 systems, namely, the part-time 4×4 system, the all wheel drive system (AWD) and the torque-on-demand (TOD) System.
In the part-time 4×4 system, the driver is allowed to select a four-wheel drive (4WD) mode, in which the front and rear driveshafts are locked together, or a two wheel drive (2WD) mode, in which the driveshafts are unlocked. In many situations, even the part-time 4×4 system provides an advantage over the 4×2 system in that the vehicle has traction if either axle has traction. For example, 4×2 (also called two-wheel-drive) vehicles may not move if there is a thick layer of snow on the road. A 4×4 vehicle can utilize the traction of all four tires to move the vehicle. During off-road driving, the vehicle may constantly meet conditions where at least one set of tires (for example the front tires) is in a low-traction situation. With 4×4 systems, the other set of driven tires can pull the vehicle out. During mountain driving, a large amount of traction may be required. A 4×4 vehicle may utilize the traction of all the four tires to overcome the gravity to move the vehicle up the hill. However, when in 4WD mode, the driveshafts are forced to rotate at the same speed. This could cause turning difficulty if the vehicle is turned at low speed on a high mu surface. In order to overcome this, all wheel drive (AWD) systems have been introduced.
An AWD vehicle has two types of differentials. The first is the axle differential. There are two axle differentials: one located between the two front wheels and one between the two rear wheels. The second is the so-called center differential, which is located inside a transfer case. The front and rear differentials enable the speed difference between the inside and outside wheels during a turn and they transfer torques from the driveshafts to the drive wheels. The center differential in the transfer case handles the speed difference between the front and rear wheels during a turn. It usually provides a constant torque split between the front and the rear axles. During normal driving and when no wheels are slipping, the center differential might direct a high amount of torque to both axles with a fixed proportion, such as 35% to the front and 65% to the rear. On low mu road surfaces, the transfer case on a part-time 4×4 system locks the front-axle driveshaft to the rear-axle driveshaft so the wheels are forced to spin at the same speed regardless if one or more wheels do not have enough traction potential. On high mu surfaces, this lock would introduce large tire slips during a turn, which could cause a jerky turn and extra tire wear. Hence, it is desired to unlock the center differential on a high mu surface and transfer torques to front and rear drive shafts in a continuously controllable fashion. This is an objective of the AWD system.
A TOD system always provides the driving torque to one axle (primary axle), and then sends torque to the other (secondary axle) as needed to provide 4WD mode while yet avoiding unnecessary differential lock in turns. In normal driving, most torque goes to the primary axle, with little torque going to the secondary axle. When in slippery conditions, if the primary axle slips, the transfer case will direct torque to the secondary axle.
In the above three types of 4×4 systems, to achieve a significant fuel economy benefit in a 2WD mode, a 4×4 system must not only disconnect the front driveline at the transfer case, but also disconnect it at the axle. This permits the front driveline to remain stationary, reducing parasitic and inertial losses. At this time no vehicles disconnect their rear-driveline, but this is an entirely practical means of improving the fuel economy of a 4×4 vehicle based on a front-wheel drive architecture. Several types of front driveline disconnect systems exist. Hublock is a system that disengages the front axle at both wheel hubs. Disconnect systems may be manual, automatic, or activated by means such as vacuum. Vacuum systems are generally considered most efficient because they disconnect the full front driveline. Center axle disconnect is a system that employs a single disconnect on one side of the axle. A free-running differential system is a system that employs a single disconnect between the differential and the ring gear of the axle. A live axle is an axle without a disconnect. If the transfer case has a disconnect or is not sending torque, the axle will effectively be backdriven from the road.
There are many choices for both axle and center differentials. Popular axle differentials used in a 4×4 vehicle might be an open differential, a passive limited-slip differential such as a Torsen differential, a viscous coupling or an electronically controlled differential. The transfer case used for an AWD vehicle might be a passive limited-slip differential or an electronically controlled viscous coupling such as a Haldex. It is desirable to consider the interaction between a brake-based roll stability control and the controlled differentials so that the interaction improves the roll stability control performance. In such a case, the system synergy by integrating those two systems enhances the individual performance compared to when the systems are separately operated.
A rule of thumb for conducting roll stability control is to reduce the excessive lateral forces at the outside wheels of the vehicle when it is driven and turned aggressively. When a vehicle is turning in response to the driver's sharp steering input, large tire forces may be induced in the lateral directions of the tires. Since the weight shifts towards the outside wheels, the outside wheels experience more lateral forces than the inside wheels. Typically, braking forces are applied to the outside front wheel to reduce the lateral force at the front outside wheel, which is the main contributor of the total lateral force applied to the vehicle body. Typically, the corrective forces in the longitudinal direction through driving torque for the purpose of reducing tire lateral forces from a 4×4 system are not directly used for achieving roll stability control performance.
It would be desirable to provide a system that uses the controlled driving torques through controlling the axle and center differentials in a 4×4 system to help reduce the potential for an on-road rollover of a vehicle, or further to provide a system that integrates the controlled driving torque from a 4×4 system control with the controlled braking torque from brake controls to achieve roll stability control performance during a potential on-road rollover event.